Lawson.] 



Geology of Carmclo Bay. 



35 



elusive on this point. No trace of olivine or of its ordinary decom- 

 position product has been detected in our slides. The chemical 

 character of the mineral is moreover not in harmony with the sug- 

 gestion of its derivation from olivine. Both calcium and sodium 

 are present in notable amounts, and it is difficult to understand how 

 these could occur in an alteration product of olivine. 



It is evidently not the form of crystallized serpentine, thermoph- 

 yllite, since it differs from the latter in physical appearance, in its 

 behavior before the blowpipe, in density, in luster, and in color; 

 neither does it correspond optically with serpentine. Moreover, 

 the development of serpentine from olivine by hydration is accom- 

 panied by a swelling of the mass. In the case of iddingsite, on the 

 contrary, there is very frequently excellent evidence of shrinkage. 

 This is clearly seen in the spaces along the cleavages, which are 

 often so open as to constitute a very large proportion of the total 

 space occupied by the phenocryst. There appears thus to be no 

 good reason for regarding the mineral as a crystallized variety of 

 serpentine. In the great majority of the many cases examined the 

 mineral is perfectly homogeneous, optically and physically. It is 

 only heterogeneous in so far as the pigment, which is not a part of 

 the silicate molecule, is unevenly distributed. It is, as Professor 

 Iddings observes, in no sense an aggregate pseudomorph. It is 

 however, not entirely free from decomposition products. In one 

 slide a few crystals were observed which had in part decomposed 

 to calcite, as was proved by treatment of the section, on the stage 

 of the microscope, by very dilute cold hydrochloric acid. The 

 material which was submitted to qualitative analysis was, however, 

 quite free from such decomposition products, and the lime was, 

 without question, part of the silicate molecule. It seems, then, that, 

 in spite of the similarity in form to olivine, there is serious doubt 

 as to its derivation from that mineral. The poverty in magnesia, 

 shown by two analyses of the carmeloi'te, strengthens this doubt. 



Two other possibilities as to the origin of iddingsite merit con- 

 sideration: (i) It may be a pseudomorph, after another mineral of 

 the same crystallographic habit as olivine, such as monticellite, or 

 (2) it may be an original separation from the magma but little 

 altered. The total absence of any trace of a non-lamellar mineral 



